The invention relates to a method for operating a digital time division multiplex (TDM) communication network, wherein subscriber terminals are connected to a central TDM trunk in a prespecified order.
A communication network in which subscriber terminals are connected in this fashion is particularly suitable if one uses a TDM trunk having a broadband transmission capacity, e.g., in the form of a glass fiber circuit, over which laser beams are transmitted as data carriers.
In a prior art communication network of this type (cf. West German Offenlegungsschrift No. 1,804,624), the basic form of the switching network comprises a number of subscriber terminals connected in a specific order to a directed transmission line which, for example, is ring-shaped. The switching in such a switching network takes place such that the subscriber terminals use a transmitted transfer address to extract the part intended for them of the total message flow routed to each subscriber (cf. NTZ Report 14, 1972, pp. 23-26). When such switching networks assume fairly large proportions and several such interconnected ring circuits are produced which are in communication with one another at a network node, or several network branches are provided that go out from network nodes (cf. NTZ Report 17, 1973, page 8), storages are available as central devices that are disposed in the network nodes only, by which one prevents message blocks entering the nodes from various directions from being unduly superimposed one another.
Certainly, the shifting of the essential switching functions to the subscriber terminals entails great expense.
Hence, in the case of a TDM communication network having a transmission circuit with broadband transmission capacity, it has been proposed to depart in the terminal area from the far-reaching decentralization of the exchange as taught in the prior art solution and to provide a central terminal exchange where, as in the prior art solution, the operation takes place with the aid of subscriber addresses transferred over the TDM trunk. According to this proposal, the communication data to be transmitted are supplied from the subscriber terminals as a result of reception of the special address transmitted from the terminal exchange in the time slot assigned to the connection in question, whereby the subscriber terminals modulate the unmodulated byte received immediately after its address in accordance with the communication data provided by them, and transfer the same in the direction of transmission.
Thus, one can in this way reduce the number of subscriber-specific devices, and synchronization problems occurring during the switching process can fully be avoided.
It is inherent that the transfer of addresses during the switching process and the maintenance of the connection in the manner indicated above require a comparatively large portion of the transmission band available for each connection. This does not have a deleterious effect, provided broadband transmission circuits are used, for example, glass fiber circuits, and as long as only telephone data shall be transmitted in such a communication network.
Particularly in view of the desire to transfer in the future types of data other than telephone data (e.g., radio data, television data in connection with video telephony, and data in communication networks), it is an object of this invention to provide a method for operating communication networks which, as far as operational reliability is concerned, at least nearly produces the same favorable results as the above teaching with respect to the full utilization of the transmission capacity of the transmission circuit, but which is only slightly less propitious than known TDM methods in which no addresses are transferred over the TDM trunk.
In accordance with the invention, the foregoing and other objects are achieved in that in a method of the type mentioned above one transmits from a central terminal exchange the communication data for the subscriber terminals participating in calls in the order of their connection to the TDM trunk. The subscriber terminals made ready for receiving such data at the start of each sampling pulse frame, yet continually blocked for the reception of communication data sent by other subscriber terminals, are disabled after receiving for the first time and supplying the communication data to be transmitted further by them for the reception of other communication data going out from the exchange. In this state the subscriber terminals allow the communication data to pass.
Thus, in known TDM methods either the allotted time slot or subscriber addresses continually accompany the communication data to enable the differentiation of the communication data pertaining to various connections. In contrast, in accordance with the invention, one uses as differentiating criteria the order in which the subscriber stations are connected to the TDM trunk or the corresponding order of the communication data intended for them.
Admittedly, a digital multiplexer arrangement has been known for transmitting digital signals between a main station and a plurality of substations (West German Auslegeschrift No. 2,015,511), wherein the substations are connected together over a single circuit. This system is so operated that the substations receive data successively from the main station and return data in the opposite direction to the main station, after which they allow further data sent out from the main station or further data returned from other substations to pass without hindrance. An exchange, however, is not provided is this known arrangement, since all the existing substations are continually involved in the connection with the main station, even though interleaved in time. Because of the type of information supplied by the individual substations, particularly in the opposite direction toward the main station, one cannot achieve the required continuity of the bit flow over the TDM trunk even if the operating mode of the known multiplexer arrangement is applied to a switching system. This is due to the different transit times between the subscriber terminals and the terminal exchanges.
A further development of the invention defines a manner in which one can establish and release a connection in a pulse code modulated/time division multiplex (PCM/TDM) switching system, how the control processes required in the subscriber terminals can conveniently be performed in connection with the preparation of the terminals for accepting communication data and the blocking or preparation for the free passage of communication data not intended therefor. It also provides a way by which different subscriber terminals can be connected to the common TDM trunks which are intended for the acceptance and output of various types of information.